PO is an essential building block for a variety of chemicals and products. Global production of PO exceeds seven million tonnes per annum.
Direct oxidation of propylene with air or oxygen to form PO tends to provide low yields. PO is therefore most commonly produced with the help of a chemical mediator.
One known process comprises contacting an organic hydroperoxide and propylene with a heterogeneous epoxidation catalyst and withdrawing a product stream comprising PO and an alcohol. A specific organic hydroperoxide that can be used in this epoxidation process is ethylbenzene hydroperoxide (EBHP), in which case the alcohol obtained is 1-phenylethanol. The 1-phenylethanol may be converted into styrene by dehydration. EBHP can be made by reaction of ethylbenzene with oxygen.
Another known process for producing PO is the co-production of PO and methyl tert-butyl ether (MTBE). This process involves similar reaction steps as the above-described styrene/PO production process. In the epoxidation step tert-butyl hydroperoxide is reacted with propylene forming PO and tert-butanol. Tert-butanol is subsequently etherified into MTBE.
Yet another known process comprises the manufacture of PO with the help of cumene. In this process, cumene is reacted with oxygen or air to form cumene hydroperoxide. Cumene hydroperoxide thus obtained is reacted with propylene in the presence of an epoxidation catalyst to yield PO and cumyl alcohol. The latter can be converted into cumene with the help of a heterogeneous catalyst and hydrogen.
Following recent advances, it is also known to produce PO from propylene with the help of hydrogen peroxide as a mediator.
Irrespective of the particular epoxidation process employed, the PO product generally requires purification to remove by-products and impurities. Indeed, for most applications, it is important to reduce impurities in PO to a very low level.
Some by-products of epoxidation processes may be readily separable by distillation. However, epoxidation processes also tend to form by-products and impurities that are more difficult to separate. Taking epoxidation with an organic hydroperoxide as an example, the organic hydroperoxide is predominantly reduced to the corresponding alcohol, which tends to be easy to separate. Also produced, however, are small amounts of other oxygen-containing compounds such as methanol, acetone, acetaldehyde, propionaldehyde and the like, as well as hydrocarbons, which are difficult to separate and often remain as impurities in the PO product even following conventional distillation. Hydrocarbon impurities associated with PO are believed to be propylene derivatives having from 4 to 7 carbon atoms per molecule, especially derivatives having 6 carbon atoms per molecule. The C6 compounds include primarily methyl pentenes and methyl pentanes. Other epoxidation processes also lead to the formation of similarly hard to separate impurities of same or similar structure.
The separation of typical impurities in PO tends to require multiple distillation steps. Furthermore, final purification (or finishing) to high levels of purity by distillation typically requires a column of very substantial size, particularly where the relative volatility of impurities compared to PO is low.
It is known to employ extractive distillation techniques to help separate impurities with a low relative volatility. For example, U.S. Pat. No. 3,909,366 describes the purification of propylene oxide by extractive distillation in the presence of an aromatic hydrocarbon having from 6 to 12 carbon atoms, such as ethyl benzene. A variety of other extractive distillation solvents have also been suggested, including for example cyclic paraffins (see U.S. Pat. No. 3,464,897), lower glycols (see U.S. Pat. No. 3,578,568), water (see U.S. Pat. No. 4,140,588), t-butyl alcohol (see U.S. Pat. No. 5,006,206) and heptane. However, such processes still typically require a column of substantial size.
U.S. Pat. No. 5,772,854 relates to the use of so-called “paired” reboilers, i.e. reboilers connected in series, in the purification of propylene oxide. Specfically, U.S. Pat. No. 5,772,854 provides a process for the purification of a propylene oxide feedstock contaminated with water, methanol and acetone in an extractive distillation column in the presence of an oxyalkylene glycol extractive distillation agent under distillation conditions selected to promote the formation and maintenance of an acetone buffer in the distillation column, wherein a higher boiling (heavier) distillation fraction containing substantially all of the oxyalkylene glycols, water, and acetone is continuously withdrawn from the distillation column and said higher boiling (heavier) distillation fraction is partially vaporized in a first reboiler; the remaining liquid being partially vaporized in a second reboiler and the vapors are recycled to the extractive distillation column.
GB-A-1549743 relates to a method for controlling the heat input to a reboiler section of a distillation column in order to enhance the separation efficiency in achieving the desired end product.
In GB-A-1549743, the “reboiler section” is defined at page 1, lines 62-64 to be that portion of the column below the lowermost tray. Thus, it will be appreciated that the so-called “reboiler section” as described in GB-A-1549743 is a bottom compartment in the distillation column.
The process of GB-A-1549743 comprises withdrawing a liquid bottoms stream from the partially partitioned reboiler section of the distillation column, introducing a first portion of liquid bottoms stream material to a first reboiler, introducing the mixed-phase bottoms stream material produced in said first reboiler to the substantially liquid-free area of the reboiler section, introducing a second portion of liquid bottoms stream material to a second reboiler, introducing mixed-phase bottoms stream material produced in said second reboiler to the same substantially liquid-free area of the reboiler section as the mixed-phase bottoms stream material produced in said first reboiler.
Thus, in process of GB-A-1549743, it will be appreciated that liquid bottoms stream material is withdrawn from the section below the lowermost tray in the distillation column and the mixed phase bottoms streams from the first and second reboilers are also returned to the same location in the distillation column, i.e. below the lowermost tray in the distillation column.
This is clearly shown in the Figure in GB-A-1549743,wherein the liquid bottoms stream material is withdrawn from the so-called reboiler section via outlet port 25, and the mixed phase bottoms streams from the first and second reboilers are returned via inlet ports 30 and 34 also in the reboiler section. The lowermost tray in the Figure is 11.
Purification of PO by distillation, and PO production as a whole, are very energy-intensive, particularly given the substantial size of the columns that are required. It is an object of the invention to provide methods and systems for separating impurities from PO that also permit energy savings to be made.